1. Field of the Invention
The present invention relates to a barrel venting system used with an extruder to remove gases generated during plasticizing, and more particularly, to such a venting system that includes a vacuum shuttle valve to control the flow of the vented gases through filtering means.
2. Description of the Related Art
When thermoplastic material is plasticized in an extruder, gases (air, water vapor, etc.) are generated in the process. Typically a vacuum is applied at the barrel of the extruder to draw off the gases since they might otherwise become entrained in the plastic melt and adversely affect the quality of the final extrusion. Removing the gases in this way causes small particles of plastic to be suspended in the flow and drawn out of the barrel with the gases. To avoid releasing the particles of plastic into the surrounding environment with the exhaust from the vacuum pump, a filtering system can be employed to separate the plastic particles from the flow of gases. In order to maintain efficient operation, such filtering systems require periodic cleaning to remove the accumulation of plastic particles from normal operation of the extruder.
It is important to maintain the vacuum on the barrel (to withdraw gases) even while the filter is being cleaned. Otherwise, the gases will become entrained in the material and the product extruded during the cleaning period will be of poor quality. One way to maintain the vacuum is to divert the flow of gases and plastic particles to the atmosphere temporarily, i.e., only as long as it takes clean the filter. A second possibility is to provide a bypass circuit for the gas flow that includes a second filter. Typically, such bypass arrangements have involved the use of two three-way valves 2 to switch the flow between the filters 4, see FIGS. 1a and 1b. Three-way valves have right angle changes in flow direction 6 and "dead end" passages 8 which tend to clog with the small plastic particles (often more like a powder) that are carried in the flow. Furthermore, in order not to interrupt the vacuum and potentially spoil a portion of the extruded product, the two valves must be operated simultaneously. However, there is no practical way to tell whether the valves were effectively operated; only close examination of the extruded product, after the fact, will tell whether the gases were properly extracted.
The prior art has failed to provide a venting system for an extruder that provides effective filtration of the gas flow and avoids potential interruption or unsatisfactory operation of the extruder while the filter is being cleaned.